- Describe animal-like protists.
- Give an overview of plant-like protists.
- Identify types of fungus-like protists.
- algae (singular, alga)
- plant-like protists such as diatoms and seaweeds
- type of protozoa, such as Amoeba, that moves with pseudopods
- type of protozoa, such as Paramecium, that moves with cilia
- type of protozoa, such as Giardia, that moves with flagella
- multicellular seaweed that may grow as large as a tree and occurs in forests found throughout the ocean in temperate and arctic climates
- protozoa (singular, protozoan)
- animal-like protists such as Amoeba and Paramecium
- slime mold
- fungus-like protist commonly found on rotting logs and other decaying organic matter
- sporozoa (singular, sporozoan)
- type of protozoa that cannot move as adults
- water mold
- fungus-like protist commonly found in moist soil and surface water
Protists are often classified based on how similar they are to other eukaryotes—animals, plants, and fungi. This lesson describes protists that resemble each of these other eukaryote kingdoms.
Animal-Like Protists: Protozoa
Animal-like protists are commonly called protozoa (singular, protozoan). Most protozoa consist of a single cell. They are animal-like because they are heterotrophs, and are capable of moving.
Ecology of Protozoa
Protozoa generally feed by engulfing and digesting other organisms. As consumers, they have various roles in food chains and webs. Some are predators. They prey upon other single-celled organisms, such as bacteria. In fact, protozoa predators keep many bacterial populations in check. Other protozoa are herbivores. They graze on algae. Still others are decomposers. They consume dead organic matter. There are also parasitic protozoa that live in or on living hosts. For example, the protozoan that causes malaria lives inside a human host. For their part, protozoa are important food sources for many larger organisms, including insects and worms.
Classification of Protozoa
Protozoa can be classified on the basis of how they move. As shown in Table below, protozoa move in three different ways. Only sporozoa cannot move. Note that this classification is based only on differences in movement. It does not represent phylogenetic relationships.
|Type of Protozoa||How It Moves||Example (Genus)|
|Sporozoan||does not move (as adult)||
Plant-Like Protists: Algae
Plant-like protists are called algae (singular, alga). They are a large and diverse group. Some algae, diatoms, are single-celled. Others, such as seaweed, are multicellular (see Figure below).
Why are algae considered plant-like? The main reason is that they contain chloroplasts and produce food through photosynthesis. However, they lack many other structures of true plants. For example, algae do not have roots, stems, or leaves. Some algae also differ from plants in being motile. They may move with pseudopods or flagella.
Ecology of Algae
Algae play significant roles as producers in aquatic ecosystems. Microscopic forms live suspended in the water column. They are the main component of phytoplankton. As such, they contribute to the food base of most marine ecosystems.
Multicellular seaweeds called kelp may grow as large as trees. They are the food base of ecosystems called kelp forests (see Figure below). Kelp forests are found throughout the ocean in temperate and arctic climates. They are highly productive ecosystems.
TED Ed: Attack of the killer algae
Classification of Algae
Types of algae include red and green algae, euglenids, and dinoflagellates (seeTable below for examples).
|Type of Algae||Origin of Chloroplast||Type of Chloroplast|
|cyanobacteria||two membranes, chlorophyll like the majority of cyanobacteria|
|cyanobacteria||two membranes, chlorophyll like a minority of cyanobacteria|
|green algae||three membranes, chlorophyll like green algae|
|red algae||three membranes, chlorophyll like red algae|
Reproduction of Algae
Algae have varied life cycles. Two examples are shown in Figure below. Both cycles include phases of asexual reproduction (haploid, n) and sexual reproduction (diploid, 2n). Why go to so much trouble to reproduce? Asexual reproduction is fast, but it doesn’t create new genetic variation. Sexual reproduction is more complicated and risky, but it creates new gene combinations. Each strategy may work better under different conditions. Rapid population growth is adaptive when conditions are favorable. Genetic variation helps ensure that some organisms will survive if the environment changes.
KQED: Algae Power
QUEST explores the potential of algae-–once considered nothing more than pond scum–-to become the fuel of the future. Entrepreneurs from throughout California are working to create the next generation of biofuels from algae. But will you ever be able to run your car off it? See http://www.kqed.org/quest/television/algae-power for additional information.
Fungus-Like Protists: Molds
Fungus-like protists are molds. They are absorptive feeders on decaying organic matter. They resemble fungi, and they reproduce with spores as fungi do. However, in other ways, they are quite different from fungi and more like other protists. For example, they have cell walls made of cellulose, whereas fungi have cell walls made of chitin. Like other protists, they have complicated life cycles with both asexual and sexual reproduction. They are motile cells during some stages of their life cycle. Two major types of fungus-like protists are slime molds and water molds.
Slime molds are fungus-like protists commonly found on rotting logs and compost. They move very slowly in search of decaying matter to eat. When food is scarce, individual cells swarm together to form a blob-like mass, like the “dog vomit” slime mold in Figure below. The mass glides along on its own secretions, engulfing decaying organic matter as it moves over it.
11 As a dog returns to his own vomit,
So a fool repeats his folly.
There are two types of slime molds when it comes to how they swarm: acellular and cellular.
- When acellular slime molds swarm, they fuse together to form a single cell with many nuclei.
- When cellular slime molds swarm, they remain as distinct cells.
Cellular slime molds are used as model organisms in molecular biology and genetics. They may be the key to how multicellular organisms evolved. Can you explain why?
Water molds are commonly found in moist soil and surface water. Many are plant pathogens that destroy crops. They infect plants such as grapes, lettuce, corn, and potatoes. Some water molds are parasites of fish and other aquatic organisms.
- Animal-like protists are called protozoa. Most consist of a single cell. Like animals, they are heterotrophic and capable of moving. Examples of protozoa include amoebas and paramecia.
- Plant-like protists are called algae. They include single-celled diatoms and multicellular seaweed. Like plants, they contain chlorophyll and make food by photosynthesis. Types of algae include red and green algae, euglenids, and dinoflagellates.
- Fungus-like protists are molds. They are absorptive feeders, found on decaying organic matter. They resemble fungi and reproduce with spores as fungi do. Examples of fungus-like protists include slime molds and water molds.
Lesson Review Questions
1. How are protozoa similar to animals?
2. What roles do protozoa play in food chains and webs?
3. State pros and cons of asexual and sexual reproduction in algae.
4. How are fungus-like protists similar to fungi? What is one way they are different?
5. Assume that a new species of organism has been discovered and it’s your job to classify it. The organism consists of a single cell with a nucleus. It has cilia and obtains food by consuming other single-celled organisms. Name a genus that the new species could possibly be placed in. Explain your answer.
6. Compare and contrast algae and plants.
7. Explain why dinoflagellates and euglenids have chloroplasts with three membranes instead of two.
Points to Consider
In this lesson you read about slime molds and water molds. These aren’t the only kinds of molds. In fact, you are probably more familiar with molds that are classified as fungi. The next lesson introduces the fungi.
- How do you think fungi might be different from fungi-like protists? (Hint: Fungi are also eukaryotes, but they belong to a different kingdom than protists.)
- What types of molds might be fungi rather than protists?
Previous: Introduction to Protists
Next: Introduction to Fungi
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